Micro Solar Array Update

While the thermo electric system continues to take most of my attention, my micro solar array has been functioning all day every day in the meantime. It started as a single 1W panel charging a 2-cell NIMH pack. It quickly grew to a 2W system at 12V charging a 4-cell NIMH pack (through a linear regulator of course). I had been running that same configuration for a few months actually. The charge current wasn’t great, but I don’t use that battery pack as much. A 2W system similar to mine is what you typically get when you buy one of those portable solar charger things for your phone or other small gadgets. The problem with them is that they are horrifically underpowered. It would take days, maybe even weeks to charge the kinds of packs those systems come with. 2W into my 2500mAH pack would take a long time. And 2W is the optimum. The average will be considerably less. I had also been frustrated with the literally weeks of zero sun we had been having. So, when the sun finally did poke out this weekend. I splurged for two more of the 1W panels. I configured the micro array of 4 panels into a 12V-12V parallel system. The results are pretty good. In full sun, at the best angle I can reasonably stay at, it was driving 150mA into the 4 cell pack. That’s very good. Even when the sun was obscured by clouds, it was still able to push in 30mA. I expect that as we continue into spring and summer, the sun will be at better angles and be stronger, so these numbers will only get better. I’ve been considering a 2 cell li-ion pack for both my solar and thermo electric systems. The pack comes complete with charge circuitry and raw power bypasses. The voltage is quite a bit higher than something I can use for 5V usb applications, so I will likely need to start investigating buck regulators. In fact, I’m considering replacing the LM317 on my solar charge circuit with a buck regulator system. At the relatively low voltages and currents the system sees, I question the efficiency gains to be had. However, as the system continues to grow (especially on the solar side. my original plans were for an 8 watt 24V system) the linear regulator is going to become more and more of a problem.